Protein Structure and Function

Give an account of protein structure and function
Protein structure
1 Proteins consist of amino acids joined together (in chains)
2 A protein is unique because of the sequence of amino acids
3 The amino acids are joined by strong peptide bonds
4 to produce the primary structure
5 Further (weak) hydrogen bonding between acids
6 produce the secondary and tertiary structures
A maximum of 4 marks can be gained from this section.

Protein function
7 Some proteins are enzymes + named example (eg the digestive enzyme amylase)
8 Some proteins are hormones + named example (eg insulin)
9 Some proteins are antibodies which help the body fight infections
10 Some proteins transport substances + named example (eg haemoglobin)
11 Some proteins provide structure + named example (eg collagen)
A maximum of 4 marks can be gained from this section.

Coherence mark (1) is given if sub-headings are used, or points placed correctly in two groups.
Relevance mark (1) is deducted if protein synthesis or factors controlling enzyme action are discussed

Give an account of enzyme function under the following headings:
(a) the specificity of enzymes; 2
(b) the role of enzymes in biochemical pathways; 4
(c) competitive and non-competitive inhibition of enzymes. 4 (10)

Each numbered point is worth 1 mark. The information in brackets is not a required part of the answer.
(a) 1 One enzyme will only react with one type of chemical
2 The chemical an enzyme reacts with is called its substrate
3 The substrate fits into the active site of the enzyme
4 The chemical produced is known as the product
A maximum of 2 marks can be gained from this section.
(b) 5 A biochemical (or metabolic) pathway is a sequence of chemical reactions in which enzymes change one metabolite into another
6 Each stage in the pathway is controlled by a specific enzyme
7 Each enzyme is produced as a result of a gene coding for it
8 If there is a mutation in the gene coding for it, an enzyme may not work properly
9 and the metabolic pathway will be disrupted
10 This is known as an inborn error of metabolism
A maximum of 4 marks can be gained from this section.
(c) 11 Competitive inhibition occurs when the inhibitor competes with the substrate for the active site of the enzyme
12 The competitive inhibitor is a very similar shape to the substrate molecule
13 Competitive inhibition can be reduced by increasing the substrate concentration
14 Non-competitive inhibition occurs when the inhibitor binds to a part of the enzyme other than the active site
15 This permanently changes the shape of the enzyme (including the active site) so that the enzyme is denatured
A maximum of 4 marks can be gained from this section.
Give an account of protein synthesis under the following headings:
(a) the role of the nucleus; (5)
(b) the role of the cytoplasm. (5)

Each numbered point is worth 1 mark. The information in brackets is not a required part of the answer.
(a) 1 DNA (in the nucleus) contains the genetic code (for the particular protein to be synthesised)
2 The DNA acts as a template for the production of mRNA
3 The DNA molecule unzips
4 as the hydrogen bonds between the bases break
5 RNA nucleotides align themselves alongside the DNA molecule
6 The RNA bases are joined to the DNA bases according to the base pair rule: A-U; C-G; G-C; T-A
A maximum of 5 marks can be gained from this section.
(b) 7 The mRNA molecule moves out of the cytoplasm and becomes attached to a ribosome
8 tRNA molecules become attached to their specific amino acids (and move to the ribosome)
9 The anticodon (triplet of bases) on the tRNA molecule
10 becomes (temporarily) attached to the complementary codon on the mRNA molecule
11 Peptide bonds form between the amino acids
12 to form a specific polypeptide (protein) molecule
A maximum of 5 marks can be gained from this section.

Give an account of the differences between the anaerobic and aerobic phases of respiration in human cells.
A total of 10 marks is available: 8 for correct facts, 1 for coherence and 1 for relevance.

Anaerobic phase
1 The anaerobic phase of respiration is called glycolysis
2 Glycolysis occurs in the cytoplasm of the cell
3 Glycolysis does not require oxygen
4 It produces 2 molecules of ATP for each molecule of glucose respired
5 The final metabolic product of anaerobic respiration is lactic acid
A maximum of 4 marks can be gained from this section.
Aerobic phase
6 The aerobic phase of respiration consists of the Krebs cycle reactions and the reactions of the cytochrome (or hydrogen carrier) system
7 The aerobic phase requires oxygen
8 The Krebs cycle reactions take place in the matrix of the mitochondria
9 The reactions of the cytochrome system take place on the christae of the mitochondria
10 The final metabolic products of the aerobic phase are water and carbon dioxide
11 Much more ATP (up to 38 molecules) is produced for each molecule of glucose respired than during the anaerobic phase.
A maximum of 4 marks can be gained from this section.
Coherence mark (1) is given if sub-headings are used, or points placed correctly in two groups.
Relevance mark (1) is deducted if any mention of the chemistry of glycoslysis, Krebs cycle or cytochrome system are discussed.
Give an account of the production and secretion of proteins under the following headings:
(a) rough endoplasmic reticulum 4
(b) Golgi apparatus; 3
(c) cell membrane. 3

(a) 1 The ribosomes found on (the surface of) the rough endoplasmic reticulum
2 synthesise proteins (for secretion)
3 The newly synthesised proteins pass into the inside of the RER
4 Here they begin to form the secondary and tertiary protein structures
5 Vesicles containing the proteins pinch off from the RER
A maximum of 4 marks can be gained from this section.
(b) 6 The vesicles from the RER transfer the proteins to the Golgi apparatus
7 In the Golgi apparatus the proteins are processed
8 for example by having carbohydrate added to them
9 Vesicles containing the processed proteins break off from the Golgi apparatus
A maximum of 3 marks can be gained from this section.
(c) 11 The vesicles from the Golgi apparatus move towards the cell membrane
12 and fuse with it
13 The proteins are secreted from the cell
14 This process is known as exocytosis
A maximum of 3 marks can be gained from this section.
Give an account of the role of lymphocytes in the immune system.
A total of 10 marks is available: 8 for correct facts, 1 for coherence and 1 for relevance.

B-lymphocytes
1 B-lymphocytes mature in the bone marrow
2 They produce specific antibodies
3 to foreign (or non-self) antigens
4 The response of B-lymphocytes is called the humoral response (because the antibodies have their effects away from the B-lymphocytes)
5 (After the initial response) memory cells remain in the body
6 The memory cells cause a faster/stronger secondary immune response (on subsequent exposure to the pathogen)
A maximum of 4 marks can be gained from this section.
T-lymphocytes
7 T-lymphocytes mature in the thymus
8 The antigens on infected cells are changed and recognised as foreign antigens by T-lymphocytes
9 The T-lymphocytes destroy the infected cells directly
10 This is known as the cell-mediated response
11 (After the initial response) memory cells remain in the body
12 The memory cells cause a faster/stronger secondary immune response (on subsequent exposure to the pathogen)
A maximum of 4 marks can be gained from this section.
Coherence mark (1) is given if sub-headings are used, or points placed correctly in two groups.
Relevance mark (1) is deducted if macrophages are discussed.
Marks for points 11 and 12 cannot be given if they have already been given for points 5 and 6.
Give an account of immunity under the following headings:
(a) naturally acquired active immunity; 6
(b) naturally acquired passive immunity. 4

(a) 1 Active immunity is naturally acquired on exposure to a pathogen
2 B-lymphocytes produce antibodies
3 which are specific to the foreign antigens
4 T-lymphocytes destroy infected cells directly
5 Memory cells are produced (by both B and T lymphocytes)
6 On subsequent exposure (to the same pathogen) a faster secondary response is initiated
7 Immunity is long lasting
A maximum of 6 marks can be gained from this section.
(b) 8 Passive immunity is short-term
9 because no antibodies are produced/the immune system is not stimulated
10 The body receives ready made antibodies
11 Examples of passive natural immunity is the passing of antibodies across the placeta from mother to foetus/passing of antibodies in breast milk from mother to baby
12 The baby is protected (from some diseases) until it can make its own antibodies
A maximum of 4 marks can be gained from this section.

Give an account of DNA structure and replication.
A total of 10 marks is available: 8 for correct facts, 1 for coherence and 1 for relevance.
DNA structure
1 DNA consists of nucleotides
2 A nucleotide is composed of a deoxyribose sugar, a phosphate group and a base
3 There are 4 bases: adenine, guanine, cytosine and thymine
4 A DNA molecule consists of two strands of nucleotides joined together and arranged in the shape of a double helix
5 The two strands are joined by the bases according to the base-pair rule: A-T and C-G
6 The bases on opposite strands are joined by hydrogen bonds
A maximum of 4 marks can be gained from this section.
DNA replication
7 The DNA molecule unwinds and unzips
8 The hydrogen bonds between the bases break
9 Free DNA nucleotides align themselves along the exposed bases according to the base pair rule
10 Hydrogen bonds form between the bases*
11 Strong chemical bonds join adjacent nucleotides together
12 Two new identical strands of DNA are produced
13 Enzymes and ATP/energy are required for this process
A maximum of 4 marks can be gained from this section.
Coherence mark (1) is given if sub-headings are used, or points placed correctly in two groups.
Relevance mark (1) is deducted if protein synthesis or meiosis are discussed.
* The mark for point 10 cannot be given if it has already been given for point 6.
Give an account of mutation under the following headings:
(a) gene mutations; 7
(b) non-disjunction. 3

(a) 1 A gene mutation is a change in the base type or sequence in a gene
2 In a substitution mutation one base is replaced by another
3 In an inversion mutation part of the base sequence is reversed
4 In a deletion mutation a base is removed
5 In an insertion mutation a base is added
6 Insertion and deletion mutations can potentially have a greater effect than substitution or inversion mutations
7 If any of these mutations occurs in a protein-coding gene, then the protein produced may be altered (or not produced at all)
A maximum of 5 marks can be gained from this section.
(b) 8 Non-disjunction (is a chromosome mutation which) causes a change in the number of chromosomes in a cell
9 Non-disjunction results from the failure of the spindle during cell division
10 If non-disjunction occurs during meiosis then gametes may be produced which will contain either too many or too few chromosomes
11 If an abnormal gamete fertilised a normal gamete, then the resulting zygote will possess too many or too few chromosomes
12 Usually non-disjunction is a lethal mutation/any embryo produced does not usually survive
13 An example of non-lethal non-disjunction mutation is Downs' syndrome in which the individual has an extra copy of chromosome 21
14 Other example: Turner's syndrome, where the individual has only one X chromosome (XO) / Kleinfelter's syndrome, where the individual has an extra copy of the X chromosome (XXY)
A maximum of 5 marks can be gained from this section.